1. Field of the Invention
This invention relates to a method of and device for controlling the temperature of molten glass and/or a glass ribbon moving through a forming chamber of the type having molten metal. More particularly, the invention relates to the use of improved lateral and longitudinal dam barriers positioned in the molten metal at selected locations to control longitudinal and lateral convection currents, respectively in the molten metal.
2. Discussion of the Prior Art and Technical Problems
The advantages of dam barriers in the glassmaking art have been recognized. For example, in U.S. Pat. No. 789,911 granted to Hitchcock, there is disclosed a glassmaking process in which a molten metal or liquid bed contained in a tank is divided into a series of compartments by stationary partitions or dam barriers. The partitions extend from the bottom of the tank to the surface of the liquid bed. In successive compartments, there is provided a metal or metal alloy having successively lower melting points. The molten metal in the compartments is maintained at a selected temperature by way of a heater. In this manner, the glass sheet, as it moves along the molten metal, is gradually cooled or annealed by contact with different portions of the molten metal.
Another type of dam barrier is disclosed in U.S. Pat. No. 3,607,199. Disclosed in the above-mentioned patent is a float process for the manufacture of float glass on a molten metal bath. Physical dam barriers are disposed longitudinally and laterally in the bath in a U-shaped form in plan view so as to form predetermined and separate backflow currents at laterally opposed sides of the glass ribbon being formed. The backflow currents flow from the downstream and progressively cooled end of the molten bath back toward the hottest or upstream end thereof flowing around and through the longitudinal barrier ends as spaced from the upstream end walls of the bath tank. The molten metal re-enters and rejoins the bath flow beneath the glass ribbon at the upstream or glass entering end to become gradually and sufficiently reheated thereby serving to remove a potential cause for affecting irregularities in the flatness of glass.
Still another type of dam barrier is disclosed in U.S. Pat. No. 3,485,617. Disclosed in the above-mentioned patent is an apparatus for the manufacture of flat glass by advancing it over a molten metal bath in ribbon form. One or more buoyant upwardly projecting barriers are provided in the molten metal for obstructing the longitudinal flow of the molten metal in the bath. The barriers are easily depressible by the advancing glass if touched thereby. The barriers also project through the exposed bath surface at the sides of the glass ribbon.
In the manufacture of a continuous glass ribbon by the float process, refined molten glass is normally moved onto a molten metal bath at a glass temperature of about 2,000.degree.F. (1095.degree.C). Within a temperature range of about 1,500.degree.F. to 2,000.degree.F. (815.degree.C. to 1095.degree.C.), the molten glass is in the plastic state and is normally sized. The glass can be sized by (1) permitting the molten glass on the molten bath to reach equilibrium thickness, e.g., about 0.270 inches (0.68 centimeters) and then cooling the glass to provide a dimensionally stable continuous glass sheet or ribbon; or (2) providing coolers about the molten glass to contain the glass in a confined area sufficient to increase the glass thickness and then cooling the glass to provide a dimensionally stable continuous glass sheet or ribbon; or (3) applying outward forces, i.e., attenuating the glass while in the plastic state to increase the glass area and then cooling the glass to provide a dimensionally stable continuous glass sheet or ribbon.
Preferably the glass is sized when the glass temperature is in the upper region of the sizing temperature range, e.g., between about 1,850.degree.F. to 2,000.degree.F. (1010.degree.C to 1095.degree.C). This is because the glass is sufficiently plastic from edge-to-edge to permit increasing or decreasing the ribbon thickness from equilibrium thickness. More particularly, if the glass temperature is in the lower region, e.g., between about 1,500.degree.F. to 1,850.degree.F. (815.degree.C. to 1010.degree.C.) the glass can have different degrees of plasticity from edge-to-edge. When this condition exists and the glass is attenuated, the resulting glass ribbon will have strain lines which make the resulting glass ribbon optically unacceptable.
As the glass is cooled through the lower region of the sizing temperature range, it is recommended that the dropping of the temperature be steady and controlled. When this is so, the glass will be uniformly cooled throughout its mass. More particularly, it is preferred that the glass temperature drop is gradual to prevent uneven cooling of the glass surfaces that can cause optical distortion.
As can be appreciated by those skilled in the art, when glass is attenuated or compressed, the glass is normally maintained in the upper region of the sizing temperature range for a longer period of time. For example, when attenuating glass, the attenuation must be graduated to prevent strain lines in the glass ribbon which causes optical defects.
In the conventional float process for manufacturing glass such as disclosed in U.S. Pat. No. 3,083,551, the problem of achieving the criteria for manufacturing glass ribbons of different thickness, e.g., a glass ribbon of equilibrium thickness and a glass ribbon having a thickness greater than or less than equilibrium thickness is not a problem. This is because the float chamber normally has a length as measured from the feed end to the exit end of approximately 200-240 feet (61-73 meters).
Since the float chamber has a length of greater than about 200 feet (61 meters) stationary dam barriers or passive dam barriers are acceptable. This is because the stationary dam barriers provide discrete compartments of molten metal on which heaters act to control the temperature of the molten metal in the compartments. When glass ribbons of thickness less than equilibrium thickness are to be manufactured, a selected number of compartments greater than the number of compartments for manufacturing glass ribbons of equilibrium thickness are maintained at the upper region of the sizing temperature range to permit gradual attenuation of the glass ribbon. The temperature of the remaining compartments are adjusted to provide a gradual, steady and controlled lowering of the temperature through the lower region of the sizing temperature range.
Although fixed dam barriers can advantageously be used in the float process of the type disclosed in the above-mentioned U.S. Pat., there are limitations when using passive dam barriers in glassmaking process where the forming chamber has length of about 10 feet (3 meters). Such a process, for example, is disclosed in U.S. Pat. application Ser. No. 483,508 filed on June 27, 1974, in the name of Gerald E. Kunkle and entitled "Manufacture of Glass."
The manufacture of glass as disclosed in above-mentioned Kunkle application includes delivery of molten glass onto the surface of a pool of molten metal and monotonically cooling the molten glass while attenuating same to form a dimensionally stable, continuous sheet of glass. The dimensionally stable, continuous sheet of glass is then lifted upward and conveyed vertically upwardly from the pool of molten metal. The glass is further monotonically cooled until it is below the annealing point for the glass. The glass is cooled in such a manner that at the time of its lifting from the molten pool of metal, it is sufficiently viscous to prevent its further attenuation while being conveyed upwardly and its major surface temperatures are sufficiently equalized so that permanent stresses are avoided in the glass.
The distance between the entrance end of the forming chamber to move the molten glass into the forming chamber and the liftoff position to vertically convey the sheet of glass upward from the pool of molten metal is preferably less than about 100 inches (2.54 meters). Using fixed dam barriers in the forming chamber are adequate to manufacture glass ribbons of one thickness. However, because the distance which the glass travels prior to liftoff is relatively short using fixed dam barriers, i.e., controlling the molten glass in compartments by coolers, has limitations. For example, merely controlling the temperature of the molten metal in the compartments with coolers does not provide adequate flexibility for maintaining the glass within the upper region of the sizing temperature range for the required period of time when changing from thick glass ribbon, e.g., glass ribbons having a thickness of about 0.270 inches (0.68 centimeters) to thin glass ribbon, e.g., glass ribbon having a thickness of about 0.100 inch (0.254 centimeters) or less or vice versa.
More particularly, when the process as described in the above-mentioned Kunkle application is employed for manufacturing flat glass of equilibrium thickness using stationary dam barriers of the type taught in the prior art, a change to manufacturing flat glass of less than equilibrium thickness requires a shutting down of the process. This is because the floor in the forming chamber has to be modified to reposition the stationary dams to manufacture flat glass having a thickness less than equilibrium thickness.
As can be appreciated, shutting down the glassmaking process is expensive because no glass is produced and because of the labor required to modify the floor of the forming chamber.
It would therefore be advantageous if dam barriers were provided that did not have the limitation of the passive dam barriers of the prior art.